1. Field of the Invention
This invention relates broadly to field of optoelectronic and photonic integrated circuits (and associated fabrication methodology) and, in particular, to integrated circuits that employ a spot-size converter for transforming mode of light from a large-size mode to a smaller-size mode or vice-versa.
2. State of the Art
Optoelectronic integrated circuits integrate photonic functionality (e.g., laser, optical detector, optical amplifier, optical modulator, optical coupler, passive coupler, passive waveguide) with electronic functionality (e.g., transistors) on a single chip. Photonic integrated circuits integrate multiple photonic functions on a single chip. Such integrated circuits typically employ active optical devices that have a tightly bound relatively small elliptical spot size. Off-chip optical elements (such as a fiber optic waveguide or free space optics) typically support a relatively large circular spot size. A spot-size converter is typically integrated on-chip between the off-chip optical element and the active optical device. For receive applications, it functions to transform the larger spot-size supported by the off-chip optical element to the smaller spot-size supported by the active optical device. For transmit applications, it functions to transform the smaller spot-size supported by the active optical device to the larger spot-size supported by the off-chip optical element.
Spot-size converters typically include a tapered design that provides a mechanism for adiabatic transformation between the larger spot-size of the off-chip element and the smaller spot-size of the active optical device. Such tapered designs may make use of lateral taper designs or vertical taper designs or both. For example, FIG. 5 shows a prior art design that employs a rib-on-rib structure, which is described in detail in Dr. Stephen Greedy, Ph. D. Theses entitled, “Advances in the Spectral Index Method for Optoelectronic Design”, 2002, available from http://www.nottingham.ac.uk/ggiemr/publications/scg_thesis.htm. At the input/output facet, the upper waveguide (or rib) is cut-off, while the underlying waveguide (or rib) supports a mode similar in profile to the off-chip optical element (e.g., fiber optic waveguide, free space optics, etc.) that interfaces thereto. At the opposite end, the upper rib supports a mode similar in profile to the active optical device that interfaces thereto. For receive applications, as the upper rib progressively widens in the direction of propagation, it becomes the dominant guiding mechanism and at some point the light shifts from the lower rib to the upper rib undergoing the desired spot-size conversion. For transmit applications, as the upper rib progressively narrows in the direction of propagation, the bottom rib becomes the dominant guiding mechanism and at some point the light shifts from the upper rib to the lower rib undergoing the desired spot-size conversion. Fabrication of this design is an improvement over prior art designs as it requires only one planar epitaxial growth and two etch steps. However, the narrowing section of the upper rib (especially the cut-off tip of the upper rib adjacent the input/output facet) is difficult to fabricate and requires precise alignment, which typically results in unacceptable coupling losses. Moreover, the lower cladding for the device is provided by a lossy cladding layer, while the upper cladding is typically provided by the interface between the converter waveguide structure and air, which is lossy as light can escape from the waveguide into the air.
Thus, there remains a need in the art for an optoelectronic integrated circuit and/or a photonic integrated circuit that employs a spot-size converter waveguide that is easy to fabricate, that does not require such precise alignment tolerances, and that provides reduced coupling losses as compared to the prior art designs.